As crash-test boasts go, Tesla is claiming a good one: The roof of its Model S electric sedan is apparently so strong that it broke a testing machine during independent validation of its government crash-test scores.

Roof-crush strength was just one of the reasons that Tesla says its electric car just came up with a five-star rating in National Highway Traffic Safety Administration testing.

Five stars is NHTSA's top rating. While other plug-in electric models have overall five-star ratings, a perfect five for each of the three test scores is relatively rare. The 2014 Ford Focus electric also is rated five stars overall and in side-crash testing, but four in rollover and frontal-crash testing. The 2014 Chevrolet Volt is rated five stars overall but has a four in frontal-crash testing. Nissan Leaf is rated four stars overall.

Analysts sound impressed.

"Clearly, (Tesla CEO) Elon Musk knows this is how an all-new type of car from an all-new automaker changes minds and wins over skeptics," says Karl Brauer of Kelley Blue Book. "If they can continue to establish new standards in areas like in-car technology and occupant protection ... they are destined to become a fully validated car company."

Tesla says the car's electric powerplant was an advantage. The car can be designed with a longer, energy-absorbing "crumple zone" in front due to no gasoline motor up front. Instead, it has storage in front, with the batteries packed underneath and the electric motors in the rear.

Tesla says it also has shown extra concern about rear-crash safety, not currently in the NHTSA test battery but to be tested by the Insurance Institute for Highway Safety industry labs. That's because the Model S has an optional rear-facing third row of seats for children. For extra protection, it added a double bumper.

Tesla says when the independent lab's testing press busted, the Model S roof was already withstanding a load of four times the car's weight without failing.

In addition, Tesla says it avoided any specific strategies to gain better test scores. "After verifying through internal testing that the Model S would achieve a NHTSA five-star rating, Tesla then analyzed the Model S to determine the weakest points in the car and retested at those locations until the car achieved five stars no matter how the test equipment was configured."